Since light passing through the atmosphere encounters a phenomenon known as dispersion, which is simply a separation of light into rays of differing wavelengths as a consequence of their respective indexes of refraction, the accuracy of any geometric and geodetic measurement made using optical techniques based on light beams or sighting paths through a given atmosphere has always been limited and influenced by a variety of atmospheric effects. These effects generally include atmospheric turbulence as well as density and temperature gradients, and other physical property variations existing in the path of a sight line or measurement beam. The refractive index of an atmosphere on a given wavelength affects the accuracy and precision of such sightings or measurements in several ways, including effectively bending the ideally straight line of sight for a projected beam or sight path away from a "true" line of sight to the target. Therefore, a pointing error is usually introduced into any measurement made with an optical instrument where at least some portion of the measurement is optically made through the atmosphere. A simplified illustration of this kind of error is shown in FIG. 1 wherein a target T located in plane T' is sighted, from an origin point O, at a location that appears to be displaced by a distance D from the truly straight line of sight TS between point O and target T. Thus, it is clear that a correction of the sighting error to determine a truly straight line of sight despite the effects of dispersion and diffusion in the atmosphere between point O and target T is needed to correct for the displacement distance D.
The prior art has included several proposals for compensating for such pointing or sighting errors. These proposals have included methods that sight to two staffs equidistant from an origin and take reciprocal sightings from both ends of the lines of sight. However, these techniques generally assume that the index of refraction is uniform along the optical paths, and remains so during the different sightings. Such an assumption is usually not valid, especially if atmospheric turbulence exists in the lines of sight. It is well known that certain forms of correction factors can be applied to partially cure the dispersion effects, but the accuracy and precision of such techniques are not sufficient for many high-precision measurement applications.
Accordingly, the art has also included techniques to minimize the physical conditions that create the variables. For example, one technique includes the use of fans to homogenize the atmospheric effects in the test area. However, micro and macro turbulence still exists along the sight path and can impose a severe limit on the pointing accuracy of measurements made using such devices.
Other techniques known in the art use two wavelengths to compensate for atmospheric refraction. The techniques using two wavelengths generally incorporate the difference between the arrival angles of the two wavelengths to obtain a correction factor to be applied in determining the true line of sight. A summary of such techniques was published in 1984 by Springer-Verlag of Berlin, Heidelberg, New York and Tokyo under the title, "Geodetic Refraction Effects of Electromagnetic Wave Propagation Through the Atmosphere," edited by F. K. Brunner. Several of the authors mentioned in this summary are referred to below (with their names being underlined and the year dates of their edited publications being shown in parentheses). Techniques, such as disclosed in the Brunner-edited publication by Khvostikov (1946), Tengstrom (1967), and Tengstrom (1977), generally include the use of interferometry, and usually do so in a time averaged manner. Even with time averaging, such techniques are still subject to the variations of atmospheric properties because averaging cannot sufficiently resolve the effects of dispersion to provide precise accuracy for the measurements. In addition to the foregoing shortcomings in degrees of accuracy achievable with these techniques, they, along with techniques that use geodetic theodolites, such as are disclosed by Vshivkov and Shilkin, Startsev and Tukh (1955); as well as techniques such as disclosed by Brein and Glissmann (1970, 1976) that require the use of two large telescopes; or techniques such as disclosed by Dyson and Williams (1972-1981) and Astheimer and McHenry (1969) that use a rotating grating to scan an image with a moving pattern of alternative transparent and opaque lines; as well as techniques such as disclosed by Mikhailov (1975) that use optical birefringence in blocks of natural quartz, inherently include set-up and operational complexities that are not desirable.
A further technique known in geodetic measurement art includes the propagation of two beam colors, each through a separate parcel of the atmosphere. In some cases, this technique may even exacerbate the above-mentioned problems associated with atmospheric variables since additional parcels of the atmosphere, and their concomitant variables, are introduced into the measurement relationships.
Other known geodetic measurement techniques are double-ended. That is, they require the source and the receiver to be located at opposite ends of the measuring area. This kind of arrangement may introduce logistic and operational problems along with the just-discussed measuring problems, so it is often preferable to user a single-ended system, provided there is no resultant loss of measurement accuracy.
Still another known technique, developed for the U.S. Government Strategic Defense Initiative (SDI) for tracking missiles, is single-ended. That is, a measuring device and a beam generator are located on the same side of the test area. This technique measures the amount of displacement undergone by a laser beam from its source to a target or measuring plane, such as that formed by a missile in space. A beam splitter directs a return beam from the target missile, and the initial beam, to a sensor which reads the average amount of displacement and analyzes it using a computer. However, this SDI technique, so far as it is generally understood, as indicated above is probably extremely expensive and difficult to set up and operate and it apparently uses continuous laser beams, so some sort of time averaging of beam displacement is most likely employed to determine average displacement of the projected and reflected beams.
It should be understood from the foregoing discussion of the prior art geodetic measuring apparatus and methods that there is still a need for more accurate means and a methods for making measurements using an optical technique that overcome a atmosphere-associated errors, yet is easy to set up and operate relative to other means and methods presently known in the art. There is also a need for a better means and method for very accurately determining a straight light beam path using optical sighting techniques, somewhat similar to those used in surveying procedures, for example, that are easily effected yet are not faulted by errors due to atmospheric conditions existing in the measuring area.